Ultrasonic Testing of Thick Steel Products Using a Synthetic Aperture Focusing Technique with a Line-Focused Probe

摘要

An ultrasonic immersion testing technique for detecting deep internal flaws in thick steel products was developed. In the new method, in order to simplify the test system, flaw images are obtained by linear scanning of a line-focused ultrasonic probe which is coupled with the test object by a local immersion method, followed by image reconstruction based on a synthetic aperture method. Conventionally, a large diameter spot-focused probe has been used for the detection of flaws far from the test surface. However, in such testing, it was difficult to apply the local immersion method because air bubbles tend to be mixed in the waterway owing to its large cross section. However, in the developed method, the waterway can be formed stably without mixing of air bubbles because the cross section can be restricted within narrow limits by use of the line-focused beam. A FBH 4 mm in diameter located 110 mm from the test surface of a cast product could be detected with a signal-to-noise ratio above 6 dB. The detectability of the developed method was the same as that in immersion testing using a spot-focused probe 50 mm in diameter with a frequency of 1 MHz.